1. Field of the Invention
The present invention relates to a method of producing thick multi-layer substrates.
2. Description of the Related Art
According to a conventional method of producing thick multi-layer substrates by printing a plurality of insulating layers on a ceramic substrate followed by firing and, then, printing a wiring pattern on the insulating layers followed by firing, it is a customary practice to fire the plurality of insulating layers and the wiring pattern all at the same temperature.
When it is attempted to produce the thick multi-layer substrate by printing a thick-film resistor on a ceramic substrate followed by firing, printing a plurality of insulating layers successively on the ceramic substrate followed by firing and printing a wiring pattern on the insulating layers followed by firing according to the above-mentioned prior art, therefore, it can be contrived to fire the thick-film resistor, the plurality of insulating films and the wiring pattern all at the same temperature.
According to the above-mentioned production method in which the temperature of firing thick-film resistor is the same as the temperature of firing the insulating layers and the wiring patterns, however, mutual diffusion and thermal stress take place between the thick-film resistor and the neighboring insulating layers due to the thermal effect in the step of firing after the thick-film resistor has been fired, resulting in a great change in the resistance of the thick-film resistor.
The above-mentioned problem can be overcome by forming an opening in all of the insulating layers, and subjecting the thick-film resistor to the laser trimming after all of the insulating layers have been fired in order to adjust the resistance as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-65464. Forming an opening in the insulating layers, however, produces dead space in which no wiring can be laid, creating another problem of an increase in substrate area.
Further, in the conventional process, another problems arises, in which, when a number of thick-film resistors are to be formed by the laser trimming, therefore, designing of the wiring pattern becomes complex and the wiring becomes too long. Moreover, the thick-film resistor is exposed in the window, causing the device to lose stability against the external environment.
Therefore, the following method can be considered; the resistance is adjusted through the whole glass insulating layers after the whole glass insulating layers have been fired. However, still problem arises in which the laser trimming is effected through the whole thick glass insulating layers, there take place absorption, scattering and reflection on the glass insulating layers and on the interfaces thereof. Therefore, the laser output must be increased to trim the thick-film resistor. However, an increase in the laser output results in an increased thermal effect to the peripheral portions that may adversely affect the peripheral wirings and circuit elements such as giving thermal stress and the like.